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If the secondary stage consist of one or more standard type gyratory crushers, with or without non-choking concaves, the radial receiving opening should be not less than 2½ to 3 times the open-side discharge setting of the primary crusher. The larger ratio should be observed for primary jaw crushers, because of the slabbing propensities of this type.
Thus, if the primary is one of the 84-in. class of jaw crushers set at 10 in., the secondary machine should be not less than a 30-in. gyratory with straight concaves, or a 36-in. size if fitted with non-choking con caves (32in. receiving opening). A primary gyratory, set at 6 in., may be followed by one or more 16-in. crushers with straight concaves, or by the 20-in. size if fitted with non-choking concaves (161h-in. receiving opening).
When the secondary stage is to consist of gyratory crushers of the reduction type, such as the Superior McCulIy reduction crusher or the New house crusher, the ratio may be somewhat lower than those for the standard machines because, for a comparable radial receiving opening, these reduction types have larger diameter crushing chambers and, hence, longer openings (as measured parallel to the spider arms. Usually from 2 to 2½ times the primary crusher setting will be satisfactory. These cylindrical-top-shell crushers, when fitted with reversible, full-curve non-choking concaves, have effective receiving openings which are considerably less than the radial distance from top-of-head to top-of concaves. This effective receiving opening must be at least as large as the maximum open-side setting at which the primary crusher will ever be operated (that is, when mantle and concaves or jaw plates are worn thin).
Fine-reduction crushers, such as the Hydrocone, also have effective receiving openings which are smaller than their full rated openings; therefore one must observe the same precaution in selecting sizes of these machines as with the reduction types mentioned in the preceding paragraph.
In selecting crushers of the roll type to follow a preceding stage the maximum permissible nip-dimension must be not les s than the maximum discharge setting of such preceding stage; and the length of roll face should be at least 4 to 5 times this setting.
Hammermills should have a minimum throat opening not less than double the setting of a preceding stage of either gyratory or jaw crusher; and the lateral throat opening should be from 4 to 5 times such setting.
Although their characteristics are not particularly favorable for secondary or reduction crushing service, jaw crushers are sometimes so used-for example, in portable and semi portable plants and in small mining installations. Usually the machines are the single-toggle or cam-actuated types, with receiving openings whose lateral dimension is at least double the dimension from top-to-top of the jaw plates. The lateral opening should be at least twice the discharge setting of the preceding stage, and when non-choking jaw plates are used the effective nip opening must be checked against this discharge setting, just as is done for the gyratory types.
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